Answer
The indications for gated myocardial perfusion single-photon emission computed tomography (SPECT) are based on its prognostic value, cost, and feasibility in virtually all patients. The prognostic value is exceptional. Although gated myocardial perfusion SPECT is more expensive than stress echocardiography, it has a better negative predictive value. This advantage makes the overall value of the tests comparable because fewer additional tests are required. The cost is also significantly less than the cost of angiography. [35, 27, 36, 37, 10, 11, 12, 13, 17, 38]
A meta-analysis of 34 studies with a combined 25,574 individuals reported an annual cardiac event rate of 1.40 and a population event risk of 2.72 following a negative SPECT result. This study provides further evidence that negative results convey an excellent prognosis and support the decision to forgo further testing. [39]
Another important advantage of nuclear scanning is that it can be performed in all patients. Many patients are excluded from treadmill testing because of osteoarthritis, poor conditioning, and amputation (among other reasons). Stress echocardiography frequently provides poor results because of the inability to obtain a good window in patients with large chest walls.
Scanning is performed for 3 reasons: (1) to aid in the diagnosis of CAD, (2) to stratify the risk in patients with known CAD, and (3) to evaluate the patient's response to therapy for CAD.
To diagnose CAD, the first step is to establish the pretest probability of disease based on patient history, physical examination findings, and laboratory data. Using this information, clinicians can generally place patients into the low-risk, intermediate-risk, or high-risk categories. Some patients with a low likelihood of disease do not need further testing; instead, only counseling on how to reduce their specific risk factors is necessary. In patients with a low-to-intermediate pretest likelihood of disease, the next step is exercise treadmill testing. If the exercise treadmill test result is negative, the patient can be safely treated by medical means, and no further testing is necessary. Patients with a positive result from the exercise stress test are good candidates for nuclear scanning, as are patients with an intermediate-to-high risk of CAD.
Stress myocardial perfusion imaging also can play an important role in the evaluation of patients presenting with acute chest pain. After an initial evaluation and diagnostic workup in the emergency department is negative, myocardial perfusion imaging performed within 48 hours of discharge appears to be effective and safe in confirming or ruling out the diagnosis of coronary artery disease. [40]
In patients with known CAD, nuclear scanning is helpful in risk stratification. With decades of clinical experience, nuclear scanning has consistently offered high clinical and economic value. Perhaps the most important finding in nuclear scanning is a negative one, with no perfusion defects or wall-motion abnormalities. Regardless of findings during catheterization, patients with a negative scan have an extremely low risk of myocardial infarction or cardiac death. [39] Patients from widely different backgrounds in whom scan findings are normal have consistently been shown to have an excellent prognosis. This observation applies whether they are male or female and old or young, whether they have positive or negative treadmill test results, and whether they have anginal or nonanginal chest pain.
In addition, nuclear scanning is helpful for follow-up observation after a coronary artery intervention. Obtaining a scan approximately 4 months after angioplasty, stenting, or bypass surgery helps determine the success of the intervention (eg, if the native artery or bypass graft remains patent). Nuclear scanning can also help determine whether a patient's chest pain results from the sternotomy or from recurrent myocardial ischemia. Because single-vessel CAD is often treated medically, nuclear scanning can also be used to ascertain whether medical treatment is reducing the ischemia.
Although myocardial perfusion SPECT imaging has several other indications, in almost all patients, detecting myocardial ischemia is the critical issue. For example, a nuclear scan after a myocardial infarction can demonstrate whether viable myocardium remains in the affected area. This information can help predict whether bypass surgery, angioplasty, or stenting will be of benefit. A nuclear scan obtained preoperatively (especially before major vascular surgery) also aids in determining risk, primarily based on the presence or absence of myocardial ischemia. Newer metabolic imaging procedures involving positron emission tomography (PET) can help determine whether the region of the heart affected by a myocardial infarction remains viable. [41] Newer fatty acid imaging serves a similar role.
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The risk of coronary artery disease (CAD) can quickly be stratified by determining whether the patient's pain is nonanginal, atypical, or typical. For men in their 30s with nonanginal chest pain, the pretest probability of disease is approximately 5%; however, men in their 30s with typical chest pain have a 70% likelihood of disease (a 14-fold increase).
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In women, the risk of CAD can quickly be stratified on the basis of the type of chest pain. For example, a woman in her 50s with nonanginal chest pain has a pretest probability of disease of approximately 10%, whereas a woman in her 50s with typical chest pain has a pretest likelihood of approximately 80%; this rate is an 8-fold increase.
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An exercise stress test is inadequate for excluding CAD when the pretest probability of disease is intermediate to high. Of 1000 American men older than 60 years who have nonanginal chest pain, the exercise stress test causes clinicians to miss almost one third of CAD cases.
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Adding nuclear cardiac imaging to the stress ECG test greatly increases the clinical value of the test. Of 1000 American men older than 60 years who have nonanginal chest pain, CAD is misdiagnosed in 25 of the 281 men who have the disease; however, of these 25 patients, the annual rate of severe cardiac events (myocardial infarction or cardiac death) is still less than 1% per year.
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Normal perfusion on a single-photon emission computed tomography (SPECT) perfusion study.
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A SPECT perfusion study in a patient with a large degree of tracer redistribution affecting the left anterior descending and right coronary artery territories.
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This study in a patient with normal findings on myocardial perfusion scanning represents the value of gated imaging in nuclear cardiac scans. The gated images show akinesis of the distal anterior wall along with a reduced left ventricular ejection fraction and an elevated end-systolic volume.
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Polar maps can help identify abnormal findings on nuclear cardiac images. The polar map shows fairly good end-diastolic perfusion, a decreased regional ejection fraction and motion by the septum, and fairly normal thickening.
